In common spark plugs, combustion is initiated from a plasma generated by striking an arc between two electrodes. With typical spark energies of 30 to 40 mJ. these devices are not capable of igniting mixtures with an air/fuel ratio significantly greater than stoichiometric.
Published work indicates that a jet of plasma is much more efficient at igniting mixtures with a high air/fuel ratio (see, for example, SAE Technical Papers 770355 and 800042). In spark plugs designed to generate jets of plasma at a high air/fuel ratio, a high energy (&gt;1 Joule) is required to be dissipated in the arc.